1. Field of the Invention
The present invention relates to an optical scanning apparatus, a method for performing the same and an image forming apparatus.
The image forming apparatus according to the present invention performs image formation by an electrophotographic process.
That is, the image forming apparatus according to the present invention can be carried out as an optical printer such as a laser printer, an optical plotter, a digital electronic copier, a plain paper facsimile, and the like.
2. Description of the Related Art
Recently, for an image forming apparatus such as a laser printer, a digital electronic copier, and a plain paper facsimile, colorization of formed images, an increase in the speed, and downsizing of the apparatus have been demanded.
In response to such demands, various image forming apparatuses that use a plurality of photoconductive photoreceptors have been proposed and realized.
As such an image forming apparatus, an apparatus that “shares a light source for optical scanning” with respect to a plurality of photoreceptors has been proposed (Japanese Patent Application Laid-open No. 2012-145667).
As a light source for optical scanning in an image forming apparatus, a “semiconductor light-emitting element” such as a semiconductor laser or a surface-emitting semiconductor laser (so-called “vertical-cavity surface-emitting later (VCSEL)”) is generally used.
High-speed drive of these “semiconductor light-emitting elements” has been realized and a signal for modulating exposure energy based on image information becomes a “modulation signal of a higher frequency” from several to several tens of megahertz.
In a semiconductor light-emitting element used for such a “modulation signal of an extremely high frequency”, “offset light emission” is always performed for light-emission rise characteristics and stability of light-emitting power.
The offset light emission means to emit light with constant weak light emission intensity during optical scanning, even during a time when light emission does not contribute to image write based on image information.
When a semiconductor light-emitting element as a light source is shared by a plurality of photoreceptors and the “offset light emission” is performed by using the shared semiconductor light-emitting element, there are following problems.
For specific explanation, there is assumed a case where “one semiconductor light-emitting element is shared by two photoreceptors”, and an image A is formed on one of the photoreceptors and an image B is formed on the other.
The images A and B are, for example, a cyan image and a black image used for forming a color image.
In this case, when both the images A and B are formed, light emission from the semiconductor light-emitting element is continuously performed, and modulation is performed alternately by a modulation signal for writing the image A and a modulation signal for writing the image B.
When only the image A is formed, the photoreceptor for the image A is optically scanned by “modulated light”; however, the photoreceptor for the image B is optically scanned by “offset light emission”.
In this case, when the photoreceptor for the image B is stopped to save energy for driving the photoreceptor, the same spot of the photoreceptor is “optically scanned repeatedly by the offset light emission”.
Therefore, light-induced fatigue occurs in a “portion which is optically scanned repeatedly” of the photoreceptor, and the photosensitive property thereof tends to be deteriorated. Such deterioration of the photosensitive property occurs “in a line shape”.
When the image B is formed by using the “photoreceptor in which the photosensitive property is deteriorated in the line shape”, an abnormal image is formed such that streaky density unevenness appears due to the deterioration of the photosensitive property in the line shape.
Therefore, there is a need to provide an optical scanning apparatus that favorably prevents the abnormal image described above.